Among reinforcing materials used for rubber compositions are galvanized steel cords, which are suitable for conveyor belts (reinforced with steel cords) working in a humid environment on account of their excellent corrosion resistance. Such galvanized steel cords are required to keep good adhesion to rubber for an extended period of time under high humid conditions so that they ensure a long life for conveyor belts.
There have been proposed several methods for achieving good adhesion between rubber and galvanized steel cords, which involve incorporation of rubber component with rosin, an organic cobalt salt, and an organic chlorine compound in combination (Patent Document 1: JP-A 2006-312744), incorporation of rubber component with any one of triazine, zinc dimethacrylate, and zinc diacrylate, and an organic cobalt in combination (Patent Document 2: JP-A 2005-350491), or incorporation of rubber component with an acid-modified polymer and zinc dimethacrylate in combination (Patent Document 3: JP-A 2006-176580).
However, the foregoing conventional technologies permit improved adhesion between rubber and galvanized steel cords but are not intended to improve rolling performance at the time of sheeting and to improve laminating performance at the time of bonding rubber layers together. Such rolling and laminating performances are important factors in industrialization that greatly affect the productivity of conveyor belts. A conceivable way of improving the rolling performance is to lower adhesion by adding stearic acid or a zinc salt of an unsaturated fatty acid. While the rolling performance in this way is improved, workability in laminating performance is greatly lowered due to great lowering of adhesion, thereby degrading laminating performance.
Consequently, there has been a demand of a new rubber composition for conveyor belts which exhibits good adhesion to galvanized steel cords and good durability in a severe working environment under humid and hot conditions and also exhibits improved rolling performance without any adverse effect on laminating performance, so that it permits efficient production of conveyor belts.